A typical torsion-elastic shaft coupling intended for use as a boat drive has a input-side coupling flange normally connected directly or indirectly through a transmission to an engine output shaft and an output-side hub connected to the shaft carrying the propeller. Two torsionally elastic coupling elements are engaged between the flange and the hub to allow for some axial misalignment between the input side and output side and to absorb torque shocks.
Highly specific demands are made on torsion-elastic shaft couplings for maritime use, especially for boat drives. Especially, where torque is low, that is at low rpm in a boat drive, the shaft coupling should be decidedly torsion-elastic, a soft coupling. With increasing torque, that is at high rpm in a boat drive, a moderate torsionally rigidity is acceptable, a stiff coupling.
If the torsional elasticity of the coupling does not adequately take into account the applied torque, the danger exists of so-called tooth chatter in the transmission. This sound is generated by the disengaging of the tooth profiles from each other and subsequent reengaging when alternating torques are greater than the load torques. This issue exists especially during motor idle and under partial load in the transition between idle and working load. Apart from the annoyance of this noise, the separation of the tooth profiles from the gear teeth leads to a significant burden on the transmission resulting in increased wear.
The use of known one-stage couplings with a linear coupling element with linear torsional rigidity is not effective because coupling elements with linear torsional rigidity are not soft enough at low torque. The use of coupling elements with progressive torsional elasticity insures acceptable operation at low torque but at high torque such coupling elements may be too stiff. The term “linear” here means that the coupling counters a low input torque with the same resistance as a large input torque. The term “progressive” here means that the coupling offers resistance that increases as the input torque increases. Thus a linear-characteristic coupling will be stiff when torque is first applied and is low, and just as stiff when the torque increases to a high level. A progressive-characteristic coupling will be soft and not transmit force at low torque, when the system is starting up or at low speed, but will be quite stiff and transmit torque fully when the torque load is great.
As a result, up to the present two- and three-stage couplings have been used in boat drives. An example of a two-stage coupling is seen in U.S. Pat. No. 5,545,089 where a first elastic coupling stage in the form of a rubber roller coupling is connected between the hub and a coupling body of a claw coupling and an outer polygon loaded coupling body. The rubber roller coupling guarantees high torsional elasticity in partial-load operation, but is not adequately dimensioned for power transmission in full-load operation. When reaching a certain torque, therefore the second coupling stage designed as a claw coupling with high torsional rigidity engages.
An example of a three-stage coupling is seen in U.S. Pat. No. 5,522,747 where the roller coupling already known from the above-described publication is used as first coupling stage. It in turn is designed for partial-load operation and is largely torsion-elastic. When it reaches a certain rated torque, a more torsionally rigid claw coupling engages that is connected to a torsion-elastic third coupling stage.
With both types of coupling, it has so far been possible to keep tooth chatter under control. Nonetheless with increasing boat drive performance both coupling concepts have reached their limits. Essentially problematic is the transition when the first coupling stage reaches its performance limit and the next coupling stage begins with power consumption. It has also been shown that there are applications in which the second stage of the two-stage coupling must be more torsion-elastic at high torques.
It must be noted that the ordering terms of “first, second, third” with reference to coupling stages or coupling elements are basically used only for conceptual differentiation and do not compellingly define the serial order with regard to the force transmission within the presented shaft couplings.